BRUSSELS — Thomson Corp.’s $18.4 billion offer for Reuters will face an in-depth inquiry by European Union antitrust regulators, who worry the deal could harm competition in the financial information industry, the European Commission said Monday.

Commission, the EU’s executive arm, said that its initial investigation revealed problems with the combination because both companies supply data feeds to traders, control news services and have access to broker research reports as well as financial information databases that they sell to their customers.

The two companies said in a statement that they would keep working with the commission “to help narrow and resolve the issues which the EC has indicated require further review.”

Thomson’s bid for Reuters Group would cut the number of major companies selling information and trading systems to the financial services industry from three — Reuters, Thomson and privately owned Bloomberg — to just two.

The EC has until Feb. 25 to decide on the deal. It rarely prevents takeovers from going ahead, usually asking companies to consider selling off units or make binding promises on how they will do business to eliminate antitrust concerns.

Apart from EU assent, the deal also requires approval from antitrust bodies in the U.K. and U.S.

Thomson and Reuters said the U.S. Justice Dept. will reveal its decision by Jan. 15.

Reuters chief exec Tom Glocer, who is CEO-designate of Thomson-Reuters, said the companies hope to complete the transaction “in or around the first quarter of 2008.”

Reuters has market share of 23%, Thomson 11%, according to April figures from Inside Market Data Reference. Bloomberg, founded by New York City Mayor Michael Bloomberg, has a 33% share.

— Associated Press